Staphylococcus aureus is a pathogen responsible for many hospital- and community-acquired infections. A critical pathogenic strategy used by S. aureus is the production of an array of bi-component, pore- forming toxins (LukSF-PV, HlgAB, HlgCB, LukED, and LukAB) that target and kill immune cells. Among these toxins, LukED has been found to be highly toxic towards both human and murine leukocytes and to contribute to S. aureus-mediated sepsis in mice. The ability of LukED to target and kill murine cells makes this toxin an excellent model to elucidate the molecular mechanism by which bi-component leukotoxins promote S. aureus pathobiology. LukED is also unique in that it targets a wide range of leukocytes including neutrophils, memory T lymphocytes, macrophages and dendritic cells. Recently the chemokine receptor and HIV co-receptor CCR5 was found to be the cellular determinant responsible for LukED-mediated targeting of T lymphocytes, macrophages and dendritic cells. Interestingly, neutrophils are highly sensitive to LukED but do not express CCR5, suggesting the presence of additional cellular receptor(s) involved in LukED-mediated targeting of these cells. We have recently identified the receptors on neutrophils that facilitate LukED-mediated targeting and killing of these cells. Interestingly, we have also identified the same set of receptors for another bi- component leukotoxin, HlgAB. Thus, the goals of this F31 application are to understand the mechanism by which LukED and HlgAB differentially recognizes cellular receptors to target and kill immune cells and to evaluate the significance of receptor recognition and targeting in vivo to S. aureus pathogenesis. Data gathered from the experiments proposed in this application will provide the necessary molecular details of how S. aureus bi-component pore-forming toxins recognize cellular determinants to target and kill immune cells involved in the protection of the host.